Carpathian Flysch Belt

The Carpathian Flysch Belt is an arcuate tectonic zone included in the megastructural elevation of the Carpathians on the external periphery of the mountain chain. Geomorphologically it is a portion of Outer Carpathians. Geologically it is a thin-skinned thrust belt or accretionary wedge, formed by rootles nappes consisting of so-called flysch - alternating marine deposits of claystones, shales and sandstones which were detached from their substratum and moved tens of kilometers to the north (generally). The Flysch Belt is together with Neogene volcanic complexes only tectonic zone occurring along the whole Carpathian arc.

Tectonic map of the Western Carpathians.

Areal extent

The Carpathian Flysch Belt is connected to the flysch belt of the Alps (Rhenodanubian Flysch) and continues through the territory of the Czech Republic, Slovakia, Poland, Ukraine and Romania. The belt is about 1,300 km long and 60 – 75 km wide.[1] Sequences of the Flysch belt are thrusted over the margin of Carpathian foredeep in the north. The foreland of the Flysch belt is built by Bohemian Massif in the west, East-European Platform in the north and Moesian Platform in the east. In the south it is bounded by the Pieniny Klippen Belt in its western segment. The southern boundary of the Flysch Belt in the area of the Romanian Carpathians is covered by nappes of the crystalline-Mesozoic zone.[2]

Geological structure

The Zone is composed basically of sedimentary rocks which were deposited since the Upper Jurassic up to Cretaceous-Paleogene time. The Flysch Belt is structural remnant of several basins, developed in front of the advancing ancestral Carpathians and later incorporated in the Tertiary Carpathian fold and thrust belt.[3] Former sedimentary basin of the Carpathian Flysch belt were portion of the Alpine Tethys Ocean. Present rocks are not in their former position because they were detached from their basement during closure and subduction of basins and pushed as nappe pile, forming Carpathian accretionary wedge. Fold axial planes have generally north vergence, North-western in the western sector, northern in the central sector and north-eastern to eastern in the eastern sector. Only nappes of South Carpathians have eastern to south-eastern vergence.

Approximately at the line of Hodonín - Námestovo - Nowy Sacz - Neresnica distinct zone of negative gravimetric anomaly that follows the southern edge of the Bohemian Massif and East-European Platform which are underthrusted below the Carpathians. Anomalous crustal thickening, significant especially in southeastern Poland and western Ukraine, is probably caused by slab break off. The Earth's crust in this area reaches up to 65 km depth.

From the neotectonic point of view whole area of flysch belt is affected by extension, locally up to 12 mm per year.

Regional division

Outer Carpathian tectonic units included in the Flysch Belt are divided according to their structural position in the frame of mountain range. Tectonic units vary not only in their structural position but also in differences in sedimentary sequences and other anomalies. Various tectonic divisions of the Flysch Belt were introduced. Generally these principal zones can be recognized:[4][5]

  • Magura nappes (Lower Cretaceous to Eocene) in Northern Carpathians
  • Krosno-Silesia nappes (Lower Jurassic to Lower Miocene) in Northern Carpathians
  • Moldavide nappes (Lower Cretaceous to Lower Miocene) in Romanian Carpathians

Evolution
Claystone interval in the Zlín beds of the Cretaceous-Paleocene age turbiditic flysch deposits of the Western Carpathian Rača Unit. Veľké Rovné, Slovakia. Pelagic claystones were deposited between the channel deposits in periods of pelagic sedimentation.

Sedimentation in the basins of the Flysch Belt is recorded since Upper Jurassic period up to Oligocene resp. beginning of Miocene. Basins of the Flysch zone were formed in Middle Jurassic — Lower Cretaceous period of post-rift subsidence. During Upper Cretaceous—Palaeocene time inversion locally occurred. In the most areas subsidence continued through Palaeocene to Middle Eocene. Synorogenic closing of the basins followed in Upper Eocene–Lower Miocene.[6] Nappes are mostly composed of turbidites - alternating sandstones and claystones.

In the past it was believed that source area of the clastic sediments supplied to the basins was built by system of linear island elevations which were parallel to the axis of the mountain chain.[7] Although such conceptions still remains, recent interpretations assume that material was supplied by submarine canyons from the adjacent shelf areas (e.g. Nesvačilka canyon).

Nappes of the Flysch Belt were thrusted due to subduction of their basement and later formed and fold and thrust belt. Character of the lithosphere in former Flysch basins (oceanic, suboceanic or continental) is a matter of debate. Deformation of the belt was gradual. Area of the Magura Basin was deformed in the Upper Oligocene to Badenian (Middle Miocene).[8] Silezian and Ždánice units were deformed in Karpatian to Lower Badenian. Moldavide Flysch was deformed since Burdigalian, especially in Sarmatian and Badenian. Internal nappes show older Upper Cretaceous deformation.[5] Subduction of the Flysch Belt basement was generally south verging, internal units were therefore thrust over the external ones from the south to north (in the Western sector) or West to East (in Eastern sector). The Tertiary shortening of Flysch Belt is approximately 130–135 km.[9] Closure of the basins was connected with motion of Inner Carpathian crustal blocks so called lateral extrusion to the East and Northeast[10] and intensive Calc-alkaline volcanism in the Carpathian internal zones.[11] Extrusion together with the movement into the “Carpathian embayment” was coeval with prominent rotation of Western Carpathian units in counter clockwise direction (up to 90°) and clockwise rotation of Eastern Carpathian units. Loading of Flysch zone nappes forced subsidence in its foreland causing formation of Carpathian foredeep. Also coeval back-arc extension occurred in Pannonian region forming a half graben system Pannonian Basin.

References
  1. Veľký, J. (Ed.), 1978, Encyclopaedia of Slovakia vol. II. E - J. Veda, Bratislava, pp. 103 (In Slovak)
  2. Dumitrescu, I., Sandulescu, M., 1974, Flysch Zone. in Maheľ, M. (Ed.) Tectonic of the Carpathian Balkan Regions. Carpathian-Balkan Association – Commission for Tectonics. Geological Institute of Dionýz Štúr, Bratislava, pp. 253-264
  3. Teťák, F., Pivko, D. & Kováčik, M., 2019: Depositional systems and paleogeography of Upper Cretaceous-Paleogene deep-sea flysch deposits of the Magura Basin (Western Carpathians). Palaeogeography, Palaeoclimatology, Palaeoecology, 533.
  4. Plašienka, D., Grecula, P., Putiš, M., Kováč, M., Hovorka, D., 1997, Evolution and structure of the Western Carpathians: an overview. In Grecula, P., Hovorka, D., Putiš, M. (Eds.) Geological evolution of the Western Carpathians. Mineralia Slovaca - Monograph, Košice, pp. 1 – 24
  5. Sandulescu, M., 1994: Overview on Romanian geology. Overview on the geology of the Carpathians. Romanian Journal of Tectonics and Regional Geology, 2, pp. 3–16
  6. Oszczypko, N., 2004, The structural position and tectonosedimentary evolution of the Polish Outer Carpathians. Przegląd Geologiczny, vol. 52, no. 8/2, pp. 780-791
  7. Maheľ, M., 1986, Geological structure of Czechoslovak Carpathians. Paleoalpine units 1. Veda, Bratislava, 503 pp. (in Slovak with English summary)
  8. Vozár, J., Vojtko, R., Sliva, Ľ., (Editors) 2002, Guide to geological excursion. XVIIth Congress of Carpathian-Balkan Geological Association. Geologický ústav Dionýza Štúra, Bratislava, 163 p.
  9. Roure, F., Roca, E., Sassi, W, 1993, The Neogene evolution of the outer Carpathian flysch units (Poland, Ukraine and Romania): kinematics of a foreland/fold-and-thrust belt system. Sedimentary Geology, Vol. 86, 1-2, pp. 177–201
  10. Nemčok, M., Pospíšil, L., Lexa, J., Donelick, R.A., 1998: Tertiary subduction and slab break-off model of the Carpathian–Pannonian region. Tectonophysics, 295, s. 307–340
  11. Lexa, J., Seghedi, I., Németh, K., Szakács, Konečný, V., Pécskay, Z., Fülöp, A., Kovacs, M., 2010, Neogene-Quaternary Volcanic forms in the Carpathian-Pannonian Region: a review. Cent. Eur. J. Geosci., 2 (3), pp. 207-270
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